Since the solubility of water in both gasoline and air decreases with a decrease in temperature, water can enter a fuel system through condensation when the atmospheric temperature changes. For example, assume a tank containing conventional gasoline contains only one gallon of fuel. Assume also that it is closed while the outside temperature is 100 degrees F with a relative humidity of 100 percent. If this tank is left sealed and the temperature drops to 40 degrees F, water will likely condense on the inside of the tank, and dissolve in the fuel. In order for enough water to condense from the air to cause gasoline-water phase separation, however, there must be approximately 200 gallons of air per gallon of fuel over this temperature drop (100 to 40 degrees). Since oxygenated fuels can hold even more water than conventional gasoline, it is even more unlikely that enough water will condense from the air to cause gasoline-water phase separation.
Water, in the form of water vapor, can dissolve in gasoline. The more humid the air, the faster the water vapor will dissolve in the gasoline. Due to chemical equilibrium, however, assuming a constant temperature, phase separation will never occur if the only source of water is from the air. Only enough water to saturate the fuel can enter the system, and no more. Water vapor, however, dissolves in gasoline very slowly, even at very high humidity. For example, at a constant temperature of 100 degrees F and relative humidity of 100%, it would take well over 200 days to saturate one gallon of gasoline in an open gasoline can (assuming the only source of water is water vapor from the air). Water absorption from the air is far slower at lower temperatures and humidity. (At a temperature of 70 degrees and relative humidity of 70%, it would take over two years to saturate one gallon of conventional gasoline in the same gasoline can.) Again, oxygenated gasoline can hold more water than conventional gasoline, and would therefore take much longer to saturate with water.